The invention relates to a method for manufacturing fibre-reinforced structures with a glass matrix.
Fibre-reinforced structures with a glass matrix have in comparison with structures with a synthetic resin matrix in particular the advantage of greater heat resistance, and also of greater resistance to external forms of attack such as oxidation, erosion, attack by acids and similar.
A method is known in which the glass forming the matrix is added to the reinforcing fibres, in particular silicon carbide or carbon fibres, in the form of a fine glass powder. The reinforcing fibres are there passed through a suspension which contains the glass powder and may also contain solvents and/or binders. In this wet method, known as "slurry technique", the reinforcing fibres are wound with the adhering glass powder on reels and then dried. A fibre layer wound in this way is then separated and processed to prepregs for the manufacture of laid fabrics. The laid fabrics made from such prepregs are then compressed under pressure and heat (of the order of magnitude of &gt;100 bar, &gt;1000.degree. C.) by so-called hot press moulding. The glass powder adhering to the reinforcing fibres thereby melts and infiltrates the reinforcing fibres. The matrix is formed in this way. There may then be imparted to the laid fabrics during the hot press moulding the form which is envisaged for the finished structure. The use of glass ceramics for the formation of the matrix is moreover also known (K. M. Prewo "The development of fibre reinforced glasses and glass ceramics", Tailoring Multiphase and Composite Ceramics, Proc. of the 21st University Conference on Ceramic Science, Pergamon Press, New York (1986), pp. 529-547; K. M. Prewo et al., Z. Ceramic Bulletin, Vol. 65, No. 2 (1986), pp. 305/306).
A method is also known in which, during the winding of the reinforcing fibres on reels, a quartz glass monofilament is wound in the dry state together with the latter as a filler. Said quartz glass monofilament forms the source for the glass matrix. The expensive slurry technique is therefore avoided (K. M. Prewo, loc. cit., p. 531; E. Fitzer "Fibre reinforced ceramic and glasses", Proc. of International Symposium on Factors in Densification and Sintering of Oxide and Non-Oxide Ceramics, 1978, Japan, pp. 650/651).
The addition of the glass fibres to the reinforcing fibres in particular in the form of fabrics in mat form as intermediate layers is also known (DE-PS 39 37 769 C1).
All the known methods require hot press moulding at pressures of the order of magnitude of &gt;100 bar and temperatures &gt;1000.degree. C. Hot press moulding methods are extremely expensive and the size of the structures which can be manufactured by hot press moulding is limited on equipment grounds.
The manufacture of glasses, in particular pure quartz glasses, by means of the colloid-gel method is also known. In this method the glass-forming raw material is provided as an extremely fine powder --SiO.sub.2 colloid--which forms a gel when mixed with water. The structures "cast" from said gel are dried and then heated to sintering temperature (Z. Glastech. Ber. 60 (1987), No. 4, pp. 125-132; Journal of Non-Crystalline Solids, 47 (1982), pp. 435-449). The addition of metal oxides to the gel in order to obtain particular glass properties is also known (Journal of Non-Crystalline Solids, 63 (1984), pp. 183-191).
The alkoxide-gel method is also known, in which mixtures of alkoxides are hydrolysed with water and a low-viscosity gel thereby formed, from which highpurity glasses and ceramics, surface coatings and thin films can be manufactured by sintering.
A method is moreover also known for the manufacture of whisker-reinforced composite bodies with glass matrix in which the whiskers are wetted with alkoxides. Small drops are produced from the wetted whiskers by spraying and dried. Extremely small droplets are thereby obtained, from which the desired structures are hot press moulded in moulds under pressure or sintered without pressure at extremely high temperatures of 1550.degree. (EP- 0 275 652 A1; Patents Abstracts of Japan, C-576, Mar. 14 1989, Vol. 13/No. 107, JP-A 63-282131 of Nov. 18 1988).
A method is also known for the manufacture of continuous strand-reinforced composite bodies in which the fibre strands (rovings) are wetted with alkoxides, dried and then sintered. Because of the very low SiO.sub.2 content in the hydrolysed alkoxides and the very high shrinkage during the drying, multiple wetting--approx. 16-fold--is necessary here and consequently--as a result also of the cost-intensive manufacture of the alkoxide gel--economic fabrication is not possible (Dr. Armin Pfeiffer "Chemical development of a composite glass body reinforced with continuous strands for application above 800.degree. C.", Dissertation 1989, Universitat Karlsruhe).
In the case of fibre-reinforced structures with a glass matrix it is necessary, as in the case of fibre-reinforced structures with a matrix of polymers, to see to it that the matrix completely fills the spaces between the fibres and completely encloses the fibres themselves. In the case of the known method in which the reinforcement fibres are impregnated with a suspension containing glass powder, glass with a grading curve of 3 to 30.mu. and above is used. The particle size of the glass therefore lies in the order of magnitude of the fibres, which in the case of SiC fibres comes to 10 .mu.m. The fibres are therefore after the impregnation by the finely ground glass held mechanically at a distance from one another, and many and large cavities are formed. This is one reason why, in the case of the known method for manufacturing fibre-reinforced structures with a glass matrix, hot press moulding under high pressure (final temperature approx. 1220.degree. C. and final pressure 10 MPa) has to be carried out, in which the softened glass is pressed between and around the fibres. The same applies to the known method in which the glass portion is introduced in the form of glass mats (woven or non-woven fabrics). Hot press moulding requires moulds which are designed for the temperatures and pressures occurring, and is therefore extremely expensive.